A recent improvement in the field of data communications and data processing has been the use of optical interconnections. As compared to electronic interconnections, optical interconnections have much broader bandwidth. An example of a system using optical interconnections is a computer, having a processor that communicates with various memory and other related computing devices via optical devices. Another example, is a computer having multiple processing elements that communicate with each other by means of an optical switch.
Optical interconnections can be divided into two categories: guided wave and free-space. Guided wave interconnections use optical fibers or integrated optical devices. However, they have the disadvantage of providing only fixed interconnections and are cumbersome when large numbers of inputs and outputs are involved. In contrast, free-space switches use the non-interactive property of photons to provide high density non-blocking interconnects.
Free-space optical interconnections are especially suitable for parallel processing applications because of the availability of more input-output ports. An example of an application for a free space interconnect is a parallel processing system, which uses arrays of optical devices to provide connections among processing elements. Light from each input is deflected by the optical device and received by the desired output.
One implementation of such a deflection type free space interconnect uses a fixed array of holographic patterns to deflect input beams. A problem with fixed holographic methods is that the hologram cannot be changed in real time.
Another type of deflection type free space interconnect uses holograms to deflect spatially separated input beams. Programmable holograms can be provided using spatial light modulators. In general, spatial light modulators are two-dimensional planar devices, which permit a light beam or electrical signal to control the transmission or reflection of light at each point on its surface. Certain types of spatial light modulators can be used to control the phase of light at each point.
One example of using a spatial light modulator for a free space interconnection is using a liquid crystal display as the spatial light modulator. The display's pixel elements are adjusted to change the phase of transmitted light. This phase change affects the direction of the output beam. A computer may be used to control individual pixel elements, which permits real time reconfiguration of holographic switching patterns. A problem with this method is that the relatively large size of the pixel elements and the spacing between them is detrimental to efficiency. Also, the pixel elements are essentially binary in the sense that the phase of the pixel elements is difficult to control other than in either of two states.
An interconnect system that uses both a fixed hologram and a spatial light modulator is described in U.S. patent Ser. No. 5,170,269, entitled "Programmable Optical Interconnect System". A first location on the fixed hologram is used to reflect a source beam to a detector, as well as to a deformable mirror device (DMD). The DMD redirects the light to a second location on the hologram, which then redirects a second beam to the detector. The hologram and DMD determine whether this second beam is in phase or out of phase with the first beam at that detector. The resulting constructive or destructive interference determines whether that detector will receive light. Thus, a distinctive characteristic of this system is the use of both a DMD and a hologram, which each provide a beam at a detector.
A need exists for a free space switch that is simple and inexpensive. It should avoid the need for a fixed hologram and permit real time programmability of an interconnection pattern. The switch should also permit a large number of interconnects.